Polyvinylidene fluoride (hereinafter referred to as "PVDF" for brevity) is a resin having excellent weathering resistance and chemical resistance. However, PVDF has disadvantages, such as that (1) the production cost is very high since it is a fluorine-type resin; and (2) immediately after extrusion from an extruder crystallization readily occurs due to its high crystallizability, resulting in formation of spherulites, and therefore PVDF sheets not orientated by stretching have poor transparence.
In order to utilize the excellent properties of PVDF, it has been proposed to laminate a film of PVDF on other transparent commercially available resins. There are various methods to produce such laminated products. Of such methods, the simplest method is a method comprising coextruding the resins to provide a laminated product, and this method permits one-step production of laminated products.
Such coextrusion to produce laminated films, for example, including a PVDF film is known as described, for example, in U.S. Pat. No. 3,647,612, and PVDF can be laminated on, for example, polymethyl methacrylate (PMMA) or polyvinyl chloride (PVC) by coextrusion. It is described, for example, in U.S. Pat. No. 3,253,060, that PVDF and PMMA have particularly high compatibility with each other and can be uniformly mixed. In addition, it is described in British Pat. No. 1,049,088 that other acrylic ester-type resins having structures similar to that of PMMA are compatible with PVDF. Therefore, the interface between the PVDF and acrylic ester-type resins is well bonded, and in laminating a PVDF layer on other resin layers, it is preferred to interpose therebetween an acrylic acid-type resin as an intermediate layer.
An attempt has been made to produce sheets having good transparency by coextrusion and lamination of PVDF and other transparent thermoplastic resins using an acrylic ester-type resin layer comprising ethyl methacrylate or methyl methacrylate as a main structural monomer as an intermediate layer. In this case, it is important to minimize formation of spherulites formed in the PVDF layer so as not to deteriorate the transparency. It is known that formation of spherulites is typically observed when molten resins are solidified at the temperature near the crystallization temperature thereof. In order to prevent such solidification at a temperature near the crystallization temperature, therefore, it is preferred that resins extruded from an extruder are rapidly cooled to temperatures much lower than the crystallization temperature. Thus, laminated products obtained by coextrusion are rapidly cooled from the surface side of PVDF where spherulites are most readily formed.
However, when fluid-like resins extruded from an extruder are taken off, for example, through the surface of a cooling roll, vibration of fluid-like resins inevitably occurs, at least to some extent. Therefore, if the cooling roll is maintained at low temperatures such that solidification occurs just when the resin comes into contact with the cooling roll, the taken off sheet lacks flatness. Thus, the solidification time is preferably set so that the resin is cooled after it comes into close contact with the cooling roll. For this purpose, in the case of PVDF, the cooling roll is usually maintained at temperatures ranging from 60.degree. C. to 125.degree. C. However, since the cooling temperature at taking off of the PVDF layer is limited, the formation of spherulites in the PVDF layer to a certain extent inevitably occurs. Furthermore, as the thickness of the PVDF layer increases, the cooling rate of PVDF in the interior of the layer lowers, resulting in the formation of large sized spherulites. Transparency is markedly reduced by the combination of an increase of thickness and the formation of large sized spherulites. Therefore, in producing transparent laminated products having a PVDF layer as a surface layer, it is preferred to reduce the thickness of the PVDF layer as thin as is practically possible.
It is also known that in laminating a sheet having good weathering properties made of PVDF, polyvinyl fluoride (PVF) or the like, on a sheet having poor weathering properties, ultraviolet absorbers (hereinafter referred to as "UVA" for brevity) can be incorporated into the sheet of PVDF, PVF or the like to protect the underlying sheet having poor weathering properties from deterioration by ultraviolet rays. In the case of laminated sheets having a PVDF surface layer, it is considered to prevent the deterioration of underlying substrate sheets due to ultraviolet rays by incorporating UVA into the PVDF surface layer. However, the amount of UVA which can be uniformly mixed with PVDF is limited. For example, if the UVA is added in a large amount, phase separation occurs, deteriorating the transparency, and the UVA which bleeds to the surface of the PVDF surface layer may flow away by rainwater or evaporate. In the case of using the UVA in an amount so as not to cause phase separation, it has been confirmed that if the thickness of the PVDF surface layer is greatly reduced to obtained a transparent laminated product as described above, the amount of the UVA contained in the PVDF layer is very small and as a result it becomes difficult to prevent the underlying resin layer from the deterioration by ultraviolet rays.